People are generally aware of the most common causes of hallucinations, like schizophrenia and a really high fever. But lesser known are the more rare causes; here are six you probably haven't heard about.

How Smart Am I? 7 Science-Backed Signs Of High Intelligence

Intelligence is notoriously hard to measure, which is why school performance isn’t always a true indication of an individual's IQ. For example, we all know the story of how Albert Einstein, one of the most gifted scientists in world history, struggled to learn the traditional way, and as a result did poorly in school. And while it’s unlikely that we are secretly undiscovered scientific geniuses, these characteristic traits could suggest that you’re actually more intelligent than you give yourself credit for.

Mental Illness

The link between mental illness and intelligence is controversial, but science has suggested that it is still worth our attention. For example, although bipolarism affects only about 2.4 percent of the world’s population, many of history’s most accomplished minds, such as Vincent Van Gogh, Emily Dickinson and Ernest Hemingway, suffered with this condition.

Although the exact reason for this link is not completely clear, one study found that a specific protein associated with memory and curiosity in mice was also linked to bipolar disorder and schizophrenia in humans. Other research suggests that the skill needed to complete mathematical equations and process information rapidly may also put individuals at risk for mania — a state of high focus and psychomotor activity that is experienced among individuals with bipolar disorder.

Using Foul Language

While we may be inclined to associate swearing with a lack of class and education, swearing is actually a sign of an intelligent mind.

A 2016 study published in Language Sciences found that using swear words is positively associated with overall verbal fluency. This means that individuals who use more curse words may have a better vocabulary, a trait associated with intelligence.

People Who Take Risks

A 2015 Finnish study found that individuals who are open to new challenges and not afraid to take risks tend to have higher intelligence. In the study, volunteers were put in a driving simulation test where they had the opportunity to either drive past yellow traffic lights or wait for the lights to turn red. Results showed that participants who made quicker riskier decisions during the simulations had more white brain matter — an area of the brain associated with cognitive function.

Being Lazy

While being lazy is not definitively linked to increased intelligence, a recent study based on 60 test subjects suggested that “non-thinkers” are more likely to be bored and therefore are more likely to part-take in sports and other forms of physical activity. Thoughtful individuals, however, may spend more time in thought and less in the gym.

"Awareness of their tendency to be less active, coupled with an awareness of the cost associated with inactivity, more thoughtful people may then choose to become more active throughout the day," wrote the study authors.

Being The Oldest Child

Another interesting theory suggested that birth order had an influence on a child’s intelligence and oldest children are likely to have a higher intelligence than their younger siblings. This may be because older children are more likely to face stricter discipline from their parents than their other siblings might.

“In particular, earlier born siblings are more likely to be subject to rules about TV watching and to face more intense parental monitoring regarding homework,” explained one study.

In addition, other research suggested that “parental subjective belief,” or the fact that parents are most likely to praise the success of the oldest child, may also serve as a self-fulfilling prophecy and cause these children to live up to their parents’ expectations.

Worrying A Lot

Ruminating, or the act of thinking about a certain situation or experience over and over again, may also be a sign of high intelligence. A 2014 study published in the online journal Personality and Individual Difference found that “verbal intelligence,” a skill rooted in problem solving, critical thinking, and abstract reasoning, was a “unique and positive predictor of worry and rumination.”

“It is possible that more verbally intelligent individuals are able to consider past and future events in greater detail, leading to more intense rumination and worry,” the researchers said, according to the Daily Mail. “Individuals with higher nonverbal intelligence may be stronger at processing nonverbal signals from individuals they interact with in the moment, leading to a decreased need to reprocess past social encounters.”

Being Popular

Aside from being good-looking and “cool,” popular kids are also able to read other people’s emotions and personality pretty well. According to a 2015 study, it’s this skill that could set popular kid’s intelligence above their peers. According to the findings, children who are emotionally intelligent and skilled at identifying what others want, think, and feel tend to climb the social ladder with greater ease than others.